1. Field of the Invention
The present invention relates to the manufacture of hydroxylapatite, useful as a human or animal implant material and for other purposes. In more detail, the present invention concerns a novel method of preparing hydroxylapatite materials that further provides readily controllable individual crystal particle sizes and surface annealing.
2. Description of the Prior Art
Hydroxylapatite, represented by the formula Ca.sub.10 (PO.sub.4).sub.6 (OH).sub.2 or the unit formula Ca.sub.5 (PO.sub.4).sub.3 (OH), is a mineralogical term for one of the inorganic constituents of the hard tissues of living bodies such as bone, teeth, etc. In chemical nomenclature, it is also known as pentacalcium triphosphate and has a theoretical calcium to phosphorus atomic ratio (Ca/P ratio) close to 1.67 and a particular physical structure as determined by X-ray diffraction analysis. As the natural mineralogical material is a primary constituent in the formation of teeth and bones of the living body, there have been considerable attempts to synthesize a sintered or fired ceramic synthetic material for use as implants and for tooth or bone replacement. A recurrent theme in many of these attempts has concerned the resemblance of the synthesized material to the natural material. This resemblance does not concern chemical purity in the sense that the material not contain impurities so much as it has centered upon material properties such as the atomic ratio of Ca/P in the synthesized material, porosity, density and thermal stability.
In one process, according to U.S. Pat. No. 4,097,935, a substantially pure hydroxylapatite in the form of a sintered ceramic having an average crystallite size in the range of 0.2-3 micrometers(um), and further characterized by the absence of pores, cleavage along smooth curved plane and the absence of birefringence, is obtained at close to the theoretical Ca/P ratio. This material is obtained by reacting ammonium phosphate with calcium hydroxide in aqueous medium while regulating the pH to 10-12 to produce an amorphous precipitate. After separating and drying, the precipitate is fired at a temperature of 1000.degree.C. to 1250.degree.C. The time period to effect sintering increases as the temperature decreases, and thus, up to 3 hours is required at the lower temperatures in this range, e.g. 1000 .degree.C. If any porosity is to be imparted, the amorphous material is mixed with organic binder which burns out during firing or mechanical holes are drilled in the sintered product. According to some literature, this material is thermally unstable, decomposing to whitlockite, also known as tricalcium phosphate.
In U.S. Pat. 4,548,959, there is reported a synthetic ceramic hydroxylapatite useful as an implant material having an atomic ratio of Ca/P of 1.67-1.69, an average crystal size of 4-20 micrometers(um), a density of 3.14-3.16 grams per cubic centimeter (g/cc) and thermal stability, wherein whitlockite is said not to be shown even after the ceramic is heated for at least one hour at 1350.degree.C. The process for preparing this material is based upon reacting calcium hydroxide with an aqueous solution of phosphoric acid in an inert atmosphere. It is necessary in this process to use a particular calcium hydroxide derived from pressure hydrating a specially prepared calcium oxide. Further, the precipitation reaction is conducted under elevated pressure and temperature in order to obtain the desired precipitate. Such process requirments are capital and energy intensive. The gelatinous precipitate is then fired at 850.degree.-1400.degree.C. for 0.5-5 hours, preferably 1250-1400.degree.C. for 1-3 hours. This, too, is energy intensive.
U.S Pat. 4,324,772 describes a continuous, two-stage process for producing hydroxylapatite by reacting aqueous solutions of calcium oxide (lime) and phosphoric acid. In the first stage, the reaction is carried out under vigorous agitation at alkaline pH (ranging from about 9.5 to about 11) whereby approximately 90% of the reaction is completed in the first stage. The reaction is continued in the second stage still under vigorous agitation by adding additional phosphoric acid sufficient to maintain the pH at about 7 to about 7.4. After flash drying the reaction product, submicron, powdered hydroxylapatite, is recovered.